Women who participate in moderate recreational physical activity exercise may reduce their risk of breast cancer by as much as 30%, says a HealthDay News article based on a study published in the June 25 online edition of Cancer.

Researchers collected data on more than 1,500 women with breast cancer and a similar number of women without the disease. All the women had taken part in the Long Island Breast Cancer Study Project.

The results show that women who exercised before or after menopause had a reduced risk of breast cancer. Women who exercised 10 to 19 hours a week had the largest benefit—about a 30% reduced risk.

The risk of breast cancer was cut with any amount of exercise. The risk reduction was seen mostly for hormone receptor-positive breast cancer, which is the most commonly diagnosed type among American women.

However, weight gain may undermine the benefit of exercise, the article says. Even among active women, gaining a significant amount of weight, particularly after menopause, increased the risk of breast cancer, negating the beneficial effect of exercise.

Lead researcher Lauren McCullough, MSPH, said the reasons that exercise is linked with a reduced risk of breast cancer aren't known. Yet it is known that maintaining a normal body weight is associated with reduced breast cancer risk.

"It is thought that a reduction in body fat results in less exposure to circulating hormones, growth factors, and proinflammatory markers, all of which have been shown to be related to breast cancer risk," McCullough told HealthDay News.